For shits sake lets simplify this.

1. Test constant hot to neutral. Constant hot is the black wire attached to both switches. One lead on the black and one on the white. Do you get 120V? If so step 2
a. If you do not get 120v your problem is before the switch.

2. Turn on the switch and test switch leg to neutral. Do you get 120V? Switch leg is the other wire on the switch.
a. If you do not get 120v throw switch in the trash and get a new one.
b. If you do get 120v the problem is either the wire underground or the light fixture.

Most pools have a deck box somewhere that the cable from the wet niche fixture terminates in. If you turn the switch on you should get proper voltage. If the spa light works fine I suspect your problem is after the GFCI.